1. Field of the Invention
The present invention relates to a photothermographic material.
2. Description of the Related Art
In recent years, in the field of films for medical diagnosis and in the field of films for graphic arts, there has been a strong desire for decreasing the amount of processing liquid waste from the viewpoints of protecting the environment and economy of space. Technology is therefore required for light-sensitive photothermographic materials which can be exposed effectively by laser image setters or laser imagers and thermally developed to obtain clear black-toned images of high resolution and sharpness, for use in medical diagnostic applications and for use in photographic technical applications. The light-sensitive photothermographic materials do not require liquid processing chemicals and can therefore be supplied to customers as a simpler and environmentally friendly thermal processing system.
While similar requirements also exist in the field of general image forming materials, images for medical imaging in particular require high image quality excellent in sharpness and granularity because fine depiction is required, and further require blue-black image tone from the viewpoint of easy diagnosis. Various kinds of hard copy systems utilizing dyes or pigments, such as ink jet printers and electrophotographic systems, have been marketed as general image forming systems, but they are not satisfactory as output systems for medical images.
Thermal image forming systems utilizing organic silver salts are known. In particular, photothermographic materials generally have an image forming layer in which a catalytically active amount of a photocatalyst (for example, silver halide), a reducing agent, a reducible silver salt (for example, an organic silver salt), and if necessary, a toner for controlling the color tone of developed silver images are dispersed in a binder. Photothermographic materials form black silver images by being heated to a high temperature (for example, 80° C. or higher) after imagewise exposure to cause an oxidation-reduction reaction between a silver halide or a reducible silver salt (functioning as an oxidizing agent) and a reducing agent. The oxidation-reduction reaction is accelerated by the catalytic action of a latent image on the silver halide generated by exposure. As a result, a black silver image is formed in the exposed region. Further, the Fuji Medical Dry Imager FM-DPL is an example of a medical image forming system using photothermographic materials that has been made commercially available.
In addition to photographic properties, physical properties of film surfaces are very important for the photothermographic material. For example, production of the photothermographic material comprises steps of coating solutions on a long roll support, drying the coated film, winding the dried film, and finishing such as slitting and cutting to provide a roll state or a sheet state. In the above steps, the material is conveyed at a high speed while being wound or unwound. Moreover, in an image forming step, the material is also conveyed at a high speed in a sheet state or a roll state. A method of providing fine surface roughness on the film surface to decrease contact area and thereby reduce conveying resistance has been generally employed for conventional photographic materials and is also effective for photothermographic materials to improve conveying suitability thereof. For example, the use of a matting agent in a photothermographic material to provide roughness on the surface is well known in the art. Japanese Patent Application Laid-Open (JP-A) No. 2001-194749 discloses the use of a poly(methyl methacrylate) particle as an organic polymer matting agent. All patents, patent publications, and non-patent literature cited in this specification are hereby expressly incorporated by reference herein. JP-A Nos. 11-311849, 2000-235242, and 2000-298326 disclose the use of an inorganic pigment particle such as silica or titanium oxide. For a conventional wet processed color positive type silver halide photographic material, the use of a monodispersed matting agent having a refractive index of from 1.50 to 1.56 to prevent degradation of granularity and sharpness during reproduction of an original image is disclosed in JP-A No. 7-287342. A matting agent is usually incorporated in an outermost layer to enhance the matte effect. The outermost layer used herein may include an outermost layer on the image forming layer side and an outermost layer on a back layer side that is opposite from the image forming layer.
However, with respect to surface properties of photothermographic materials, there exist many specific problems that are not encountered in the conventional wet processed photographic materials. A coating solution for an image forming layer including an organic silver salt, a silver halide emulsion, and a developing agent (reducing agent) has no setting ability. The “setting ability” used herein means a property such that a solution has low viscosity and some fluidity when the temperature is about 40° C. or higher, but gelation occurs and the solution loses fluidity when it is cooled to about 17° C. or lower. Therefore, when an image forming layer and a non-photosensitive layer on the image forming layer are coated so as to be superposed, matting agent particles incorporated in the non-photosensitive layer are liable to sink into the image forming layer, so that sufficient roughness on the surface cannot be provided, because no matting agent particles remain on the surface after drying. Moreover, because photothermographic materials are heated to 100° C. or higher for forming an image, organic polymer matting agent particles are thermally deformed, so that the matting agent particles cannot provide sufficient roughness. Furthermore, because all components necessary for image formation are included in the film in advance, the film surfaces tend to adhere between overlapping films.
Furthermore, the use of such a polymer matting agent in a photothermographic material has often led to the following problem, depending on the kind and addition amount of the matting agent used. When observing a transparent image, which is obtained after exposure by a laser beam and thermal development, on a film monitor (Shaukasten), interference of a transmitted light causes glare on the image surface, so that image tone differs depending on the viewing direction.
As discussed above, the matting agent used in the photothermographic material is related to many particular properties. Therefore, selection and development of the most suitable components are very important for improving overall performances of the photothermographic material, and therefore, development of more excellent components has been eagerly demanded.
On the other hand, in order to improve film properties, polymer latex is generally mixed in a hydrophilic binder such as gelatin in the photothermographic material.
Concerning the polymer latex used for the above purpose, preferred is a polymer latex having a refractive index which is close to that of gelatin and having no light scattering property in the film. However, the demand described above has not yet been sufficiently fulfilled.